1. Technical Field
The present disclosure relates to flash memory, and more particularly, to a flash memory device and a smart card including the same.
2. Discussion of the Related Art
Semiconductor memory devices are generally classified as volatile or nonvolatile according to whether memory can be retained in the absence of power. Random access memories such as SRAM and DRAM are typically volatile memory devices. Nonvolatile memory devices include read-only memories (ROMs), e.g., erasable and programmable ROMs (EPROMs), electrically erasable and programmable ROMs (EEPROMs), and flash memories.
Modern flash memory devices have small size, low power consumption, and advanced reading/writing performance. For example, flash memory devices are usually employed in providing on-chip memory system for portable apparatuses such as cellular phones, digital cameras, audio/video recorders, modems, smart cards, and so forth. Such portable apparatuses require quick access to data. For example, in smart cards that must be secured against unauthorized access. Modern intrusion and authentication circumvention techniques may be used to monitor internal operation of smart cards by detecting consumption and variation of currents and electromagnetic fields that are present during internal activation. Accordingly, security features may be added to smart cards to protect against the monitoring of currents and electromagnetic fields within the smart card.
A flash memory cell, e.g., a split-gate flash memory cell uses F-N tunneling for erasing data thereof and source-side channel hot electron injection for programming data therein. To program a memory cell using source-side channel hot electron injection, a word line of the selected memory cell is driven with a voltage about 1.2V and a source line of the selected memory cell is driven with a voltage about 9V. For data to be programmed (program data), a bit line of the selected memory cell may be driven with a voltage about 0.3V. Accordingly, current is consumed as it flows from the bit line to the source line through the selected memory cell. For data to be inhibited against programming (program-inhibited data), a bit line of a selected memory cell may be driven by a power source voltage turning off the selected memory cell to interrupt a current to the hit line from the source line.
As discussed above, in a secure integrated circuit card equipped with a flash memory device having flash memory cells, high voltages over a power source voltage are normally used for writing or erasing specific information. A high voltage generator for generating high voltages is usually inefficient and an amount of current dissipated during a programming operation may depend upon the number of data bits to be programmed. For example, an amount of current dissipated in simultaneously programming 32 memory cells may differ from the amount of current dissipated in simultaneously programming 16 memory cells. Because of this, there may be an observable drop in current consumption as memory cells are accessed. This observable drop in current consumption may be a security vulnerability.